In the field of factory automation, a building-block type programmable logic controller including a plurality of control units is conventionally used. In the programmable logic controller in which the control units are mounted to a base unit, the mounting positions of the control units are set in such a manner that adjacent ones of the control units are placed with no space therebetween.
In the programmable logic controller, upper limits are set on the ambient temperature during the operation and the altitude of the use place to ensure the proper operation in accordance with a control program. If the operation in an environment exceeding these upper limits is forced, heat is not sufficiently dissipated from a control circuit and the like in the control unit and the programmable logic controller gets into a state in which a malfunction is possibly caused due to overheating.
For example, in a programmable logic controller including control units of reference standard which are designed, assuming a predetermined operating environment, if a configuration modification is applied to enhance the heat dissipation property or suppress the amount of heat generation, the upper limit on the ambient temperature and the upper limit on the altitude can be raised while basic specification of reference standard is maintained. In order to enhance the heat dissipation property, there are measures to be taken such as placing the control units at every other slot in the base unit, adding a unit for applying a forced cooling such as a fan, and adding a heat dissipation device and the like. As a measure to be taken to suppress the amount of heat generation, limiting the function of the control units and the like can be considered.
Furthermore, for example, Patent Literature 1 discloses a housing of a module-type controller in which the housing has an external shape obtained by narrowing the entire portion from the central part of right and left side surfaces that constitute the housing toward the front face side. Patent Literature 1 describes that the heat dissipation property is enhanced by causing air to communicate with an internal space via through holes formed in the side surfaces of the housing.